The invention relates to cutter assemblies for rotary drill bits for use in drilling or coring holes in subsurface formations.
The cutter assemblies are for use in rotary drill bits of the kind comprising a bit body having a shank for connection to a drill string, a plurality of cutter assemblies mounted at the surface of the bit body, and a passage in the bit body for supplying drilling fluid to the surface of the bit for cleaning and/or cooling the cutters. Each cutter assembly comprises a preform cutting element mounted on a carrier.
One common form of preform cutting element comprises a tablet, for example circular, having a thin, hard cutting layer of polycrystalline diamond bonded to a thicker, less hard backing layer of cemented tungsten carbide. The preform cutting element is then mounted on the carrier, for example by a process known as "LS bonding."
The carrier, which is usually generally cylindrical in shape, is received in a socket in the surface of the bit body. The bit body itself may be machined from metal, usually steel, or may be molded using a powder metallurgy process. In known cutter assemblies of this type, it has been usual for the carrier to be formed from cemented tungsten carbide which has characteristics which render it particularly suitable for this purpose. Thus, it exhibits high rigidity, high resistance to the erosion to which such carriers are subject in use, and hot strength. Also, the coefficient of expansion of tungsten carbide is sufficiently close to the coefficient of expansion of polycrystalline diamond to reduce the residual stresses which can occur when the two materials are bonded together. However, some of the other characteristics of cemented tungsten carbide have certain disadvantages.
For example, cemented tungsten carbide has low toughness (i.e. it is comparatively brittle) and this can lead to failure of such cutter assemblies in use, as a result of impact forces on the assembly. Also, after prolonged use, a large wear flat develops on the carrier and bears on the formation being drilled. Due to the high abrasion resistance of the tungsten carbide, this leads to high heat generation due to friction, with consequent overheating and premature failure of the polycrystalline diamond layer of the preform cutting element. The combination of low toughness and high heat generation also causes heat checking of the tungsten carbide carrier material with resultant premature failure of the bit. "Heat checking" is a term of art which refers to craze cracking of the wear flat which develops on the carrier due to abrasive heating with intermittent quenching by the drilling fluid.